The invention relates to a system for digital encoding of analog television signals utilizing discrete sampling signals.
In television signal processing it is often desirable to separate various signal components from each other, such as for image enhancing or color signal processing.
For example, where it is necessary to separate the luminance and the chrominance signals of a composite color television signal such as for recording, it is important to avoid penetration of any portion of the luminance signal into the chrominance signal. If a remaining small portion of the luminance signal is processed together with the chrominance signal, a disturbing flicker is generated which will appear in the reconstructed television picture. A preferable technique which yields a desirable clean signal separation is so-called comb filtering, which is well known in the art. One way of utilizing the comb filtering technique is to selectively add and subtract picture elements of three adjacent color television lines in a given field, to obtain the chrominance and luminance signals. This particular technique is performed by sampling and forming a weighted average of three vertically aligned picture elements from three adjacent horizontal lines of the television field raster and it is repeated for all picture elements. The comb-filtering technique is particularly advantageous for digitally encoded color television signals such as by pulse code modulation (PCM) where each individual code corresponds to a particular amplitude value of the analog signal at a particular sampling time. There are known techniques for digitally encoding color television signals at a sampling frequency which is an even or odd multiple of the NTSC color subcarrier frequency in preparation for subsequent comb filtering. When the sampling frequency is, for example, an even multiple of the NTSC subcarrier frequency, the samples on subsequent lines utilized for PCM encoding are separated by exactly one horizontal line period and thus they are vertically aligned on the television raster. However, if an odd multiple of the subcarrier frequency is utilized as the sampling signal frequency, only the samples on alternate horizontal lines are vertically aligned whereas the samples on adjacent lines are misaligned with respect to each other by one-half sample period due to the well known relationship between the subcarrier and horizontal line frequency of NTSC signals. This relationship is defined as the color subcarrier frequency, f.sub.SC, being an odd multiple of one-half horizontal line frequency, f.sub.H, and is given by the equation f.sub.SC = 455/2 f.sub.H. Therefore, if comb filtering is to be utilized for signals sampled at an odd multiple of subcarrier frequency, it is necessary to provide additional means to produce vertical alignment of sampling signals on all horizontal lines of the television field raster. One way of solving this particular problem is described for example in a prior art U.S. Pat. No. 3,946,432 to Goldberg et al. where sampling signals of three-times the NTSC color subcarrier frequency are generated and inverted for alternate lines. The non-inverted and inverted sampling signals are used during alternate lines as a clock signal for sampling the analog color television signal for digital encoding thereof. The result is that the television signal is sampled line by line at vertically aligned points of the television raster. However, the above-mentioned prior art encoding technique has disadvantages which will be discussed below.
When encoding an analog color television signal into discrete signal levels at high operating speeds, it is essential that vertical alignment of samples be maintained within very close tolerances of approximately 1 to 2 nanoseconds over a period of at least three consecutive lines to prevent hue errors and ensure effective comb filtering. By generating two oppositely phased square wave sampling signals, such as in the above-indicated patent, it is difficult to achieve alignment of the corresponding transitions of these signals within the required close tolerances, and thus generate sampling signals at accurate equidistant sampling intervals. For example, when utilizing ordinary digital logic components, there is an inherent variation between rise and fall times of the generated square wave. When such a square wave is inverted, the rise and fall times thereof may differ from those of the non-inverted form of the signal, thus introducing relative asymmetry between the two signals. Furthermore, when considering that for generation of oppositely phased sampling signals two separate signal channels are utilized, it is apparent that there will be timing errors introduced by the differences in signal propagation through the respective channels due to different time delays effected by variations in the circuit components utilized in the respective channels. To improve the vertical alignment of the respective samples formed by the inverted and non-inverted sampling signals it is necessary for the waveform applied to the signal inverter of the prior art encoder to have an extremely low even order harmonic content, otherwise the previously mentioned asymmetry between the non-inverted and inverted waveforms occurs. To compensate for such problem would complicate the apparatus and increase its cost. Furthermore, there will be a difference in noise pickup in the respective inverted and non-inverted signal channels thus contributing additional signal asymmetry.
There is still an additional significant disadvantage of the above-mentioned prior art color television signal encoding technique in that only a sampling signal frequency which is an integral multiple of the subcarrier frequency may be utilized for signal encoding by that technique. Thus, for encoding of other than NTSC television signals where the frequency relationship between the color subcarrier and horizontal line frequency is different and somewhat more complicated than in NTSC, such as in PAL and other systems, the above-mentioned prior art technique is not applicable. It is an object of the present invention to provide a circuit for digital encoding of television signals which overcomes the above-described and other disadvantages of the prior art systems.